Laser sintering is a solid imaging process for building three-dimensional objects, layer-by-layer, from a working medium utilizing sliced data sets representing cross sections of the object to be formed. Typically an object representation is initially provided by a computer aided design (“CAD”) system. The laser sintering system dispenses a thin layer of heat-fusible powder, normally a fusible polymer powder, polymer coated metal, metal, or ceramic, over a bed of the powder commonly referred to as the “part cake.” The laser sintering system applies thermal energy to melt those portions of the powder corresponding to a cross-section of the article being built in that powder layer. Lasers typically supply the thermal energy through modulation and precise directional control to a targeted area of the powder layer. Conventional selective laser sintering systems use carbon dioxide lasers and position the laser beam by way of galvanometer driven mirrors that deflect the laser beam.
The part cake typically includes a movable build platform upon which the bed of powder is disposed. After a powder layer is fused, the build platform moves downward by an incremental distance. The system then dispenses an additional layer of powder onto the previously fused layer and repeats the process of melting and selective fusing of the powder in the next layer, with fused portions of later layers fusing to fused portions of previous layers as appropriate for the article, until the article is complete. These articles are sometimes referred to as “objects,” “parts,” or “builds;” and the “part cake” includes not only the build(s) but the unfused powder surrounding the build(s). Each additional layer of powder typically is dispensed from a powder feed system that dispenses a measured amount of powder on the part cake. A powder spreader, such as a blade or roller, then spreads the powder over the part cake bed in a uniform manner. In many older systems, once the build is made, it remains within the process chamber under an inert atmosphere until cooled. A newly formed build may require several hours or days to cool and, as a result, the laser sintering system may be inactive during the cooling time, which may cause the system to be unavailable for subsequent builds.
Later laser sintering devices have been developed that include a removable build chamber in which the build platform and build are contained. The build chamber including the new build may be separated from the process chamber so that the part can be cooled outside the process chamber. Meanwhile, a fresh build chamber can be inserted and a new build can be prepared without waiting for the previous build to cool.
Various attempts have been made for more rapid cooling of the part cake outside the laser sintering system to increase the efficiency and speed with which builds can be produced. However, rapid cooling can distort the build(s) within the part cake, so the speed of cooling the part cake must be carefully controlled. It would be desirable to develop an alternative way to rapidly cool part cakes so that the laser sintering system continues to be used to produce new builds during cooling of prior builds and while minimizing distortion of the build and maximizing recovery of unused laser sintering powder for reuse.
In addition, the part cake (including the build) can be discolored if the part cake is removed from the laser sintering system while the part cake is above a certain temperature, for example 150° C., as the result of oxidation of the build and powder. The discoloration can permeate many layers deep into the part cake relative to the outermost layer exposed to ambient conditions; therefore, conventional laser sintering systems typically require that the part cake remain in the laser sintering system a certain amount of time simply to prevent discoloration of the build.